U.S. patent application number 13/530822 was filed with the patent office on 2013-12-26 for metal nanowire networks and transparent conductive material.
The applicant listed for this patent is Melburne C. LeMieux, Ying-Syi Li, Ajay Virkar. Invention is credited to Melburne C. LeMieux, Ying-Syi Li, Ajay Virkar.
Application Number | 20130341074 13/530822 |
Document ID | / |
Family ID | 49773463 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130341074 |
Kind Code |
A1 |
Virkar; Ajay ; et
al. |
December 26, 2013 |
METAL NANOWIRE NETWORKS AND TRANSPARENT CONDUCTIVE MATERIAL
Abstract
Metal nanowires, such as silver nanowires coated on a substrate
were fused together to form fused metal nanowire networks that have
greatly improved conductivity while maintaining good transparency.
Materials formed form the fused metal nanowire networks described
herein can have a transparency to visible light of at least about
85% and a sheet resistance of no more than about 100 Ohms/square or
a transparency to visible light of at least about 90% and a sheet
resistance of no more than about 250 Ohms/square. The method of
forming such a fused metal nanowire networks are disclosed that
involves exposure of metal nanowires to various fusing agents on a
short timescale. When formed into a film, materials comprising the
metal nanowire network demonstrate low sheet resistance while
maintaining desirably high levels of optical transparency, making
them suitable for transparent electrode formation.
Inventors: |
Virkar; Ajay; (Mountain
View, CA) ; Li; Ying-Syi; (Fremont, CA) ;
LeMieux; Melburne C.; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Virkar; Ajay
Li; Ying-Syi
LeMieux; Melburne C. |
Mountain View
Fremont
San Jose |
CA
CA
CA |
US
US
US |
|
|
Family ID: |
49773463 |
Appl. No.: |
13/530822 |
Filed: |
June 22, 2012 |
Current U.S.
Class: |
174/255 ;
174/250; 174/257; 427/98.4; 977/773; 977/810 |
Current CPC
Class: |
H05K 2201/0108 20130101;
H05K 3/1283 20130101; H05K 1/097 20130101; B82Y 30/00 20130101;
H05K 2201/026 20130101 |
Class at
Publication: |
174/255 ;
174/250; 174/257; 427/98.4; 977/773; 977/810 |
International
Class: |
H05K 1/03 20060101
H05K001/03; H05K 3/00 20060101 H05K003/00; H05K 1/02 20060101
H05K001/02; H05K 1/09 20060101 H05K001/09 |
Claims
1. A material comprising a transparent conductive coating and a
substrate on which the coating is supported, the coating comprising
a fused metal nanowire network comprising fused metal nanowires,
wherein the coating has a transparency to visible light of at least
about 85% and a sheet resistance of no more than about 100
Ohms/square or a transparency to visible light of at least about
90% and a sheet resistance of no more than about 250
Ohms/square.
2. The material of claim 1 wherein the metal nanowires have an
aspect ratio from about 50 to about 5000 and a diameter of no more
than about 250 nm.
3. The material of claim 1 wherein the metal nanowires have an
aspect ratio from about 100 to about 2000 and a diameter from about
10 nm to about 120 nm.
4. The material of claim 1 wherein the metal nanowires comprises
silver, copper, gold, indium, tin, iron, titanium, platinum,
palladium, nickel, cobalt, or an alloy combination thereof.
5. The material of claim 1 wherein the metal nanowire comprises
silver nanowires.
6. The material of claim 1 wherein the metal nanowires on the
substrate has a surface loading level that is about 0.1
.mu.g/cm.sup.2 to about 5 mg/cm.sup.2.
7. The material of claim 1 wherein the substrate comprises glass,
polymer, inorganic semiconducting material, inorganic dielectric
material, laminates thereof, composites thereof or combinations
thereof.
8. The material of claim 7 wherein the polymer comprises
polyethylene terephthalate (PET), polyacrylate, polyolefin,
polyvinyl chloride, fluoropolymer, polyamide, polyimide,
polysulfone, polysiloxane, polyetheretherketone, polynorbornene,
polyester, polyvinyl alcohol, polyvinyl acetate,
acrylonitrile-butadiene-styrene copolymer, polycarbonate, a
copolymer thereof or blend thereof.
9. The material of claim 1 further comprising a polymer film
overcoat.
10. The material of claim 1 having a sheet resistance of no more
than about 75 ohm/sq and a transparency of at least about 85% at
550 nm.
11. The material of claim 1 having a sheet resistance of no more
than about 175 ohm/sq and a transparency of at least about 90% at
550 nm.
12. A method of forming a transparent, electrically conductive
film, the method comprising: depositing a plurality of metal
nanowires as a coating on a surface of a substrate to form a
pre-treatment material; and exposing the pre-treatment material to
a vapor fusing agent for no more than about 4 minutes to fuse at
least some of the metal nanowires together to form the transparent
electrically conductive film that comprises fused metal nanowire
network.
13. The method of claim 12 wherein the fusing agent comprises vapor
of HCl, HBr, HF, HI or combinations thereof.
14. The method of claim 12 wherein the exposing to the fusing agent
is performed for no more than about 3 minutes.
15. The method of claim 12 wherein the metal nanowires comprises
copper, gold, tin, iron, titanium, indium, platinum, palladium,
nickel, cobalt, or an alloy combination thereof.
16. The method of claim 12 wherein the metal nanowires comprise
silver nanowires.
17. The method of claim 12 wherein the metal nanowires has a
surface loading level on the substrate that is about 0.1
.mu.g/cm.sup.2 to about 5 mg/cm.sup.2.
18. The method of claim 12 wherein the fused metal nanowire network
of the treated material has a transparency to visible light at 550
nm of at least about 85% and a sheet resistance of no more than
about 100 Ohms/square.
19. The method of claim 12 wherein the fused metal nanowire network
has a transparency to visible light at 550 nm of at least about 90%
and a sheet resistance of no more than about 250 Ohms/square.
20. A method of forming a transparent electrically conductive film,
the method comprising: depositing a dispersion of metal nanowires
onto a substrate surface; delivering a solution comprising a fusing
agent in a solvent onto the substrate surface; and drying the
substrate surface after depositing the metal nanowires and
delivering the fusing agent solution to fuse at least some of the
metal nanowires into the transparent electrically conductive film
comprising a fused metal nanowire network.
21. The method of claim 20 wherein the fusing agent comprises HCl,
HBr, HF, LiCl, NaF, NaCl, NaBr, NaI, KCl, MgCl.sub.2, CaCl.sub.2,
AlCl.sub.3, NH.sub.4Cl, NH.sub.4F, AgF, or a combination
thereof.
22. The method of claim 20 wherein the solution has a halide ion
concentration from about 0.1 mM to about 10 M and wherein the
solvent comprises an alcohol, water, or a combination thereof.
23. The method of claim 20 wherein the metal nanowire dispersion
further comprises the fusing agent such that the depositing of the
nanowire dispersion and the fusing agent solution are performed
simultaneously.
24. The method of claim 20 wherein the delivering of the fusing
agent solution is performed after depositing the metal nanowire
dispersion.
25. The method of claim 20 wherein the metal nanowires comprises
copper, gold, tin, iron, titanium, indium, platinum, palladium,
nickel, cobalt, or an alloy combination thereof.
26. The method of claim 20 wherein the metal nanowires comprise
silver nanowires.
27. The method of claim 20 wherein the metal nanowires has a
surface loading level on the substrate that is about 0.1
.mu.g/cm.sup.2 to about 5 mg/cm.sup.2.
28. The method of claim 20 wherein the fused metal nanowire network
of the treated material has a transparency to visible light at 550
nm of at least about 85% and a sheet resistance of no more than
about 100 Ohms/square.
29. The method of claim 20 wherein the fused metal nanowire network
has a transparency to visible light at 550 nm of at least about 90%
and a sheet resistance of no more than about 250 Ohms/square.
30. A device comprising at least one transparent electrode having a
transparent conductive material comprising a fused metal nanowire
network of claim 1.
Description
FIELD OF THE INVENTION
[0001] The invention relates to fused networks of metal nanowires
that are suitable for the formation of electrically conductive and
transparent films, such as for use as transparent electrodes. The
inventions are further related to chemical methods for fusing the
nanowires to form networks as well as to devices incorporating the
fused metal nanowire networks.
BACKGROUND
[0002] Functional films can provide important functions in a range
of contexts. For example, electrically conductive films can be
important for the dissipation of static electricity when static can
be undesirable or dangerous. Optical films can be used to provide
various functions, such as polarization, anti-reflection, phase
shifting, brightness enhancement or other functions. High quality
displays can comprise one or more optical coatings.
[0003] Transparent conductors can be used for several
optoelectronic applications including, for example, touch-screens,
liquid crystal displays (LCD), flat panel display, organic light
emitting diode (OLED), solar cells and smart windows. Historically,
indium tin oxide (ITO) has been the material of choice due to its
relatively high transparency at high conductivities. There are
however several shortcomings with ITO. For example, ITO is a
brittle ceramic which needs to be deposited using sputtering, a
fabrication process that involves high temperatures and vacuum and
therefore is relatively slow and not cost effective. Additionally,
ITO is known to crack easily on flexible substrates.
SUMMARY OF THE INVENTION
[0004] In a first aspect, the invention pertains to a material that
comprises a transparent conductive coating and a substrate on which
the coating is supported. The coating has fused metal nanowire
networks that comprise fused metal nanowires. The coating in
general has a transparency to visible light of at least about 85%
and a sheet resistance of no more than about 100 Ohms/square or a
transparency to visible light of at least about 90% and a sheet
resistance of no more than about 250 Ohms/square. In some
embodiments, the metal nanowires have an aspect ratio from about 50
to about 5000 and a diameter of no more than about 250 nm. In
additional embodiments, the metal nanowires have an aspect ratio
from about 100 to about 2000 and a diameter from about 10 nm to
about 120 nm. The metal nanowires can comprise silver, copper,
gold, indium, tin, iron, titanium, platinum, palladium, nickel,
cobalt, or an alloy combination thereof. In some embodiment, the
metal nanowire comprises silver nanowires. The metal nanowires on
the substrate can have a surface loading level that is about 0.1
.mu.g/cm.sup.2 to about 5 mg/cm.sup.2. The substrate used can be
glass, polymer, inorganic semiconducting material, inorganic
dielectric material, laminates thereof, composites thereof or
combinations thereof. In some embodiments, polymeric substrate used
can be polyethylene terephthalate (PET), polyacrylate, polyolefin,
polyvinyl chloride, fluoropolymer, polyamide, polyimide,
polysulfone, polysiloxane, polyetheretherketone, polynorbornene,
polyester, polyvinyl alcohol, polyvinyl acetate,
acrylonitrile-butadiene-styrene copolymer, polycarbonate, a
copolymer thereof or blend thereof. In some embodiments, the
material further comprises a polymer film overcoat. In some
embodiment, the material has a sheet resistance of no more than
about 75 ohm/sq and a transparency of at least about 85% at 550 nm.
In other embodiments, the material has a sheet resistance of no
more than about 175 ohm/sq and a transparency of at least about 90%
at 550 nm.
[0005] In a second aspect, the invention pertains to a method of
forming a transparent, electrically conductive film. The method
comprises the steps of depositing a plurality of metal nanowires as
a coating on a surface of a substrate to form a pre-treatment
material; and exposing the pre-treatment material to a vapor fusing
agent for no more than about 4 minutes to fuse at least some of the
metal nanowires together to form the transparent electrically
conductive film that comprises fused metal nanowire network. The
fusing agent can be a solution of HCl, HBr, HF, LiCl, NaF, NaCl,
NaBr, NaI, KCl, MgCl.sub.2, CaCl.sub.2, AlCl.sub.3, NH.sub.4Cl,
NH.sub.4F, AgF, or a combination thereof. The solution can have a
halide ion concentration from about 0.1 mM to about 10 M in a polar
solvent, an alcohol, and or water solvent. In some embodiment, the
fusing agent can be vapor of HCl, HBr, HF, HI or combinations
thereof. The exposing to the fusing agent step of the method in
general is performed for no more than about 3 minutes. Metal
nanowires comprises silver, copper, gold, tin, iron, titanium,
indium, platinum, palladium, nickel, cobalt, or an alloy
combination thereof can be used in the method. In some embodiment,
the silver nanowires is used to form the film. The metal nanowires
on the substrate can have a surface loading level that is about 0.1
.mu.g/cm.sup.2 to about 5 mg/cm.sup.2. In some embodiments, the
fused metal nanowire network of the film has a transparency to
visible light at 550 nm of at least about 85% and a sheet
resistance of no more than about 100 Ohms/square. In other
embodiments, the fused metal nanowire network has a transparency to
visible light at 550 nm of at least about 90% and a sheet
resistance of no more than about 250 Ohms/square.
[0006] In a third aspect, the invention pertains to a method of
forming a transparent electrically conductive film. The method
comprises the steps of depositing a dispersion of metal nanowires
onto a substrate surface, delivering a solution comprising a fusing
agent in a solvent onto the substrate surface; and drying the
substrate surface after depositing the metal nanowires and
delivering the fusing agent solution to fuse at least some of the
metal nanowires into the transparent electrically conductive film
comprising a fused metal nanowire network. The fusing agent
comprises HCl, HBr, HF, LiCl, NaF, NaCl, NaBr, NaI, KCl,
MgCl.sub.2, CaCl.sub.2, AlCl.sub.3, NH.sub.4Cl, NH.sub.4F, AgF, or
a combination thereof. The solution of the fusing agent has a
halide ion concentration from about 0.1 mM to about 10 M and a
solvent that comprises an alcohol, water, or a combination thereof.
In some embodiment, the metal nanowire dispersion further comprises
the fusing agent such that the depositing of the nanowire
dispersion and the fusing agent solution are performed
simultaneously. In some embodiment, the delivering of the fusing
agent solution is performed after depositing the metal nanowire
dispersion. Metal nanowires comprises silver, copper, gold, tin,
iron, titanium, indium, platinum, palladium, nickel, cobalt, or an
alloy combination thereof can be used in the method. In some
embodiment, the silver nanowires is used to form the film. The
metal nanowires on the substrate can have a surface loading level
that is about 0.1 .mu.g/cm.sup.2 to about 5 mg/cm.sup.2. In some
embodiments, the fused metal nanowire network of the film has a
transparency to visible light at 550 nm of at least about 85% and a
sheet resistance of no more than about 100 Ohms/square. In other
embodiments, the fused metal nanowire network has a transparency to
visible light at 550 nm of at least about 90% and a sheet
resistance of no more than about 250 Ohms/square.
[0007] In a fourth aspect, the invention pertains to a device that
comprises at least one transparent electrode that uses a
transparent conductive material comprising a fused metal nanowire
network described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1A is a schematic drawing of metal grid based
transparent electrode formed through a traditional patterning
approach.
[0009] FIG. 1B is a schematic drawing of a nanowire (NW) based
transparent conductive material fabricated from low cost solution
processable methods disclosed herein.
[0010] FIG. 1C is a schematic drawing illustrating the process of
three nanowires being fused together to form an elongated nanowire
with two angles around the fused points.
[0011] FIG. 1D is a schematic drawing illustrating nanowires being
fused together to form a fused NW based transparent conductive
material with angles around the fused points and arrows indicating
the formation of NW network.
[0012] FIG. 2 is a plot of sheet resistance of the samples from the
first vendor tested before and after the HCl vapor treatment having
a transparency at 550 nm greater than 75%.
[0013] FIG. 3 is a plot of sheet resistance of the samples from the
second vendor tested before and after the HCl vapor treatment
showing dramatic improvement in conductivity.
[0014] FIG. 4 is a plot of sheet resistance of the samples tested
before and after the HCl vapor treatment having a transparency at
550 nm greater than 85%.
[0015] FIG. 5 is a scanning electron micrograph (SEM) of silver
nanowires before any treatment.
[0016] FIG. 6 is a SEM micrograph of silver nanowires after heat
treatment.
[0017] FIG. 7 is SEM micrographs of fused silver nanowires after
HCl vapor treatment.
[0018] FIG. 8 is a plot of sheet resistance of samples treated with
5 mM NaCl in ethanol and AgF in ethanol solutions.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] A fused metal nanowire network can be formed chemically to
achieve a structure with desirably low electrical resistance and
high transparency to visible light. The fused metal nanowire
network can be formed as a coating for use as a transparent
conductive layer. Silver nanowire can be a convenient material to
form the network, but other metal nanowires are also suitable for
forming the network of fused metal nanowires. The chemical fusing
can be performed using gas phase or solution phase ionic inorganic
compositions with halogen anions. Since the electrically conductive
network can be formed at low temperature, the networks are suitable
for use with materials, such as polymers, that cannot tolerate high
temperatures. Thus, the conductive networks are well suited to
certain transparent electrode applications and the low quantities
of materials and low temperature processing can provide for
convenient commercial applications.
[0020] Metal nanowires can be formed from a range of metals. For
example, the production of a range of metal nanowires is described,
for example, in published U.S. patent application 2010/0078197 to
Miyagishima et al., entitled "Metal Nanowires, Method for Producing
the Same, and Transparent Conductor," incorporated herein by
reference. There has been particular interest in silver nanowires
due to the high electrical conductivity of silver. With respect to
the specific production of silver nanowires, see for example,
published U.S. patent application 2009/0242231 to Miyagisima et
al., entitled "Silver Nanowire, Production Methods Thereof, and
Aqueous Dispersion," and U.S. 2009/0311530 to Hirai et al.,
entitled "Silver Nanowire, Production Method Thereof, and Aqueous
Dispersion," and U.S. Pat. No. 7,922,787 to Wang et al., "Methods
for the Production of Silver Nanowires," all three of which are
incorporated herein by reference. Silver nanowires are commercially
available, for example, from Seashell Technologies, LLC, CA,
USA.
[0021] Silver is known to have a bulk melting point of about
960.degree. C. However, nanoparticles of silver can melt at
temperatures less than 150.degree. C. This melting point depression
observed for the nanoparticles are believed to be based on the
large surface area/volume ratios of the nanoparticles. In general,
the larger the surface area/volume ratio, the greater the expected
mobility of the surface atoms, and the lower the melting point.
Melting points of about 150.degree. C. for silver nanoparticles
however may still be too high for a variety of substrates including
plastics and elastomers. The time required for melting and cooling
can also be in excess of several minutes, which add time and
process costs to production.
[0022] To produce flexible transparent conductive material that can
be produced at reasonable cost and in large scale such as
roll-to-roll coating or ink-jet printing method, numerous new
materials have been developed as replacements for indium tin oxide
(ITO). A potential ITO replacement is a metal-grid shown in FIG.
1A. Metal grids, which can be formed using patterning approaches
such as photolithography, can achieve very high performances with
low sheet resistances. However, the metal grid films are not
solution-processable for example with roll-to-roll coating and
therefore are costly to fabricate and often involve fabrication
methods which are difficult to scale. While the performance of
metal grids may exceed ITO, cost and processability are still
hindering their wide-spread adoption.
[0023] As shown schematically in FIG. 1B, the metal nanowires
deposited into a film from a dispersion can appear to be randomly
arrayed rods that intersect with each other randomly, although in
practice some alignment of rods can take place depending on the
deposition process. While metal nanowires are inherently
electrically conducting, the vast majority of resistance in the
silver nanowires based films is believed to due to the junctions
between nanowires. To improve the properties, it has been proposed
to embed the metal nanowires in a secondary electrically conductive
medium, see published U.S. patent application 2008/0259262 to Jones
et al., entitled "Composite Transparent Conductors and Methods of
Forming the Same," incorporated herein by reference. In principle,
the junction resistance of a AgNW network can be reduced by
sintering or fusing the wires together using heat as disclosed in
"Modeling the melting temperature of nanoparticles by an analytical
approach." by A. Safaei et. Al. in J. Phys. Chem. C, 2008, 112,
99-105 and in "Size effect on thermodynamic properties of silver
nanoparticles" by W. Luo et al. in J. Phys. Chem. C, 2008, 112,
2359-69. The heat can be applied conventionally or by a light
source. However, conventional heating may not be practical for many
applications since the NWs are not expected to melt until
300-400.degree. C., which is significantly greater than the thermal
stability limits of most plastic substrates. Light sources can also
be used, but may involve setup of additional and expensive
equipment in a large roll-to-roll fabrication. A room or low
temperature process which fuses the NWs is therefore highly
desirable. Hu et al. disclosed similar results in ACS Nano, Vol 4,
No. 5, 2010, 2955-2963 entitled "Scalable coating properties of
flexible, silver nanowire electrodes." Hu et al demonstrated that
the junction resistance between the silver nanowires can be in the
giga-ohm range, but with processing to 110.degree. C. with the
optional addition of significant pressures for short times improved
electrical conductivity performance could be obtained.
[0024] Low temperature in combination with the application of
pressure has been used to achieve a significant decrease in
electrical resistance while reasonable levels of transparency were
reported. See, De et al., "Silver Nanowire Networks as Flexible,
Transparent Conducting Films: Extremely High DC to Optical
Conductivity Ratio," ACS Nano Vol. 3(7), pp 1767-1774 (June 2009).
The De et al. article does not suggest that fusing of the silver
nanowires takes place, and the low temperature used in the process
would seem to be too low to result in fusing. The process in the De
et al. article involved vacuum filtering and transfer using
100.degree. C. and significant amounts of pressure for 2 hours.
This process is not desirable from a commercial processing
perspective.
[0025] As described herein metal nanowires are fused at room
temperature for transparent conductive material applications using
a chemical approach. Silver nanowires in particular has been found
can be fused together to improve the sheet resistance of the film
formed from the 10.sup.5-10.sup.8 .OMEGA./sq range to the 10 to 100
.OMEGA./sq range with less than 0.5% changes to the transparency.
Nanowire network thicknesses can be used that provide overall good
transparency of at least about 85% for networks with the reported
low electrical resistance. The fusing can be achieved in less than
a minute that impose little or no change or damage to the
morphology of the metal nanowires. Thus, the process is well suited
to efficient and relatively inexpensive commercial processing.
[0026] It was recently demonstrated by Magdassi and co-workers that
thick films of silver nanoparticles (AgNPs) can be "sintered" at
room temperature using various chemical agents for non-transparent
patterned silver paste application. A process for the chemical
fusing of metal nanoparticles is described in published PCT
application WO 2010/109465 to Magdassi et al., entitled "Process
for Sintering Nanoparticles at Low Temperatures," incorporated
herein by reference. The nanoparticle low temperature sintering is
further described in Grouchko et al., "Conductive Inks with a
"Built-In" Mechanism That Enables Sintering at Room Temperature,"
ACS Nano Vol. 5 (4), pp. 3354-3359 (2011). The fusing of
nanoparticles forms a sheet of metal, which can have a desired low
electrical resistance, but the sheet of metal generally does not
have desired amounts of transparency.
[0027] A vapor based process for the formation of a conductive film
from silver nanowires is described in Liu et al., "Silver
Nanowire-Based Transparent, Flexible and Conductive Thin Film,"
Nanoscale Research Letters, Vol. 6(75), 8 pages (January 2011)
(hereinafter "the Liu article"). The films formed as described in
the Liu article had reasonably low electrical resistance, but the
transparency of the films was not satisfactory for many
applications. The Liu article attributed their observations to the
removal of surface oxidation from the silver nanowires. However,
significant deterioration of the nanowire morphology has been
observed in the micrographs shown in the Liu article. Improved
processing leads to significantly improved results for the fused
metal nanowire networks described herein. In particular, desired
levels of fusing have been achieved with short time processing of
the nanowires with the halide anions without degrading the level of
optical transparency and with little deterioration of the nanowire
morphology.
[0028] For transparent electrode applications, higher-aspect ratio
structures like wires or tubes are advantageous since the rod like
shape can promote electrical conductivity primarily in-plane. The
primarily in-plane conductivity in these rod-like structures allows
for "open" areas and thin films which are useful for high light
transmission and good 2D sheet conductivities. Nanowires (NWs) are
particularly good candidates for transparent conductor
applications. However due to their much larger size of about 10s of
nanometers in diameter and 10s-100s of microns in length, the
surface area/volume ratio of nanowires is considerably smaller than
NPs. Silver NWs for example typically do not melt until the
temperatures of about 300-400.degree. C. Silver nanowires are
10.sup.4-10.sup.5 times larger in volume relative to nanoparticles
and have much smaller ratio of surface area to volume and ratio of
surface atoms to bulk atoms. The significant differences in
physical size of nanowires relative to nanoparticles imply that the
properties are likely to be correspondingly different.
[0029] The improved fused metal nanowire networks described herein
can achieve simultaneously desirably low sheet resistance values
while providing good optical transmission. In some embodiments, the
fused metal nanowire networks can have optical transmission at 550
nm wavelength light of at least 85% while having a sheet resistance
of no more than about 100 ohms/square. In additional or alternative
embodiments, the fused metal nanowire networks can have optical
transmission at 550 nm of at least 90% and a sheet resistance of no
more than about 250 ohms/sq. Based on the ability to simultaneously
achieve good optical transparency and low sheet resistance, the
fused metal nanowire films can be used effectively as transparent
electrodes for a range of applications. The loading of the
nanowires to form the network can be selected to achieve desired
properties.
[0030] To achieve the desirable properties of the fused metal
nanowire networks, it was surprisingly discovered that short time
exposure of silver nanowires to halide containing fusing agents
could dramatically improve the conductivity of the nanowire
networks or films. In general, the metal nanowire networks can be
exposed to the fusing agent for times of no more than about 4
minutes to cause the desired fusing, and in some embodiments
significantly less time can be used as described further below. The
dramatic reduction in sheet resistance may partially be attributed
to the removal of the insulating capping polymer
polyvinylpyrrolidone (PVP) that is used to stabilize commercial
silver nanowires, but is believed to be primarily related to the
fusing of the nanowires. SEM studies of the treated silver
nanowires indicated clearly the formation of fusing points between
the nanowires that are in close proximity of each other as well as
significantly reduced amount of detectable PVP polymer. In
comparison, the SEM of the untreated silver nanowires clearly shows
the presence of PVP polymer and the gap between the ends of the
closely situated silver nanowires. Referring to FIG. 1C, a
schematic diagram illustrating the process of the ends of three
adjacent nanowires being fused together is shown. The fused
nanowires form an elongated nanowire with two angles around the
fused points. Elongated nanowires can further form a network of
elongated nanowires as shown in FIG. 1D, with angles around the
fused ends and arrows indicating the connection formation between
the elongated nanowires to form the nanowire network.
[0031] The fused silver nanowires disclosed herein have
considerable differences from the sintered silver nanoparticles
disclosed by Magdassi and the treated silver nanowires disclosed by
Liu. Specifically, after the sintering process, the AgNPs of
Magdassi aggregated together. The profiles of the individual AgNPs
that existed prior to the sintering process have been destroyed
considerably during the sintering process to form the aggregates.
The word sintering indeed is a proper description of the melting
and coalescing, and or coarsening of the silver nanoparticles of
Magdassi. With regard to the treatment of silver nanowires proposed
by Liu, although Liu intended to improve conductivity of the silver
nanowires by removal of AgO, the prolonged HCl treatment disclosed
by Liu caused observable thinning and shortening of the silver
nanowires that seems to have degraded the properties of the
resulting material.
[0032] In contrast to the processing approach described by Magdassi
et al., the current processing approach is directed to the
production of networks with a high level of optical transparency.
The processing conditions are designed to achieve this objective,
and the nanowire morphology is conducive to processing to obtain a
desired degree of transparency. In particular, conductive films can
have an optical transparency evaluated for convenience at 550 nm
light wavelength of at least about 85%.
[0033] The processing of the metal nanowire networks described
herein comprises the contact of a thin metal nanowire layer, i.e.,
a network, with a chemical fusing agent comprising a halide anion.
The fusing agent can be delivered as a vapor or in solution. For
example, acid halides are gaseous and can be delivered in a
controlled amount from a gas reservoir or as vapor from a solution
comprising the acid halide. Halide salts can be dissolved in
solution with a polar solvent with a moderate concentration, and a
quantity of the salt solution can be contacted with the nanowire
network to fuse adjacent nanowires. Suitable solvents for forming a
solution with the chemical fusing agent include, for example,
alcohols, ketones, water, or a combination thereof. It has been
discovered that superior properties of the fused network results
from short processing times for the nanowire network with the
fusing agent. The short processing times can be successful to
achieve very low levels of sheet resistance while maintaining high
optical transparency.
[0034] While the processing conditions are designed to produce good
optical transparency, the metallic grid-like properties of the
fused elongated silver nanowires dramatically increased
conductivity with little change in transparency relative to the
unfused networks. The drop in electrical resistance may be due to a
drop in junction resistance between adjacent nanowires due to
fusing of the adjacent nanowires. The treatment described herein
may also have improved the connection between the other connecting
points indicated by the arrows in FIG. 1D by removing oxidation
layer of the nanowires, by removing the capping agent such as PVP
of the nanowires, or by at least partially fusing these connection
point together. Although removal of the PVP from the surface of
silver nanowires have been observed in the examples below, fusing
of the silver nanowires at points of contact can also be observed.
The final transparent conductive material can best be described as
a fused network of silver nanowires, as illustrated in FIG. 1D. The
fused metal nanowire network structure has advantages over
conventional metal grids described in FIG. 1A due to low cost
fabrication methods and solution processability.
[0035] In summary, a highly conductive and transparent material was
formed at room temperature by fusing the ends of silver nanowires
to improve the conductivity without sacrifice the transparency. The
resulting material appears to be a silver metallic grid like
structure that is highly conductive. The examples below described
using HCl as the fusing agent from the vapor phase, dilute
solutions of HCl, NaCl, and AgF were also used to create the
materials of comparable properties at room temperature. It is
understood the metal nanowires could be treated multiple times to
achieve the desired degree of fusing, with the same or different
fusing agent during each treatment. Although silver nanowires were
used to perform the fusing experiments, it is understood that other
metal nanowires can be similarly fused together to form materials
with improved conductivity.
Electrically Conductive Film Structure and Properties
[0036] The conductive films described herein generally comprise a
substrate and a fused metal nanowire network deposited on the
substrate. An optional polymer coating can be placed over the metal
nanowire network to protect and stabilize the fused nanowire
network. The parameters of the metal nanowires can be adjusted to
achieve desirable properties for the fused network. For example, a
higher loading of nanowires can result in a lower electrical
resistance, but transparency can decrease with a higher nanowire
loading. Through a balance of these parameters, desirable levels of
electrical conductivity and optical transparency can be achieved.
The nanowires in the improved networks are fused, as is observed in
scanning electron micrographs. It is believed that the fusing of
the nanowires results in the improved electrical conductivity while
maintaining high levels of optical transparency. Having a network
with fused nanowires should provide a stable electrically
conductive structure over a reasonable lifetime of a corresponding
product.
[0037] In general, the nanowires can be formed from a range of
metals, such as silver, gold, indium, tin, iron, cobalt, platinum,
palladium, nickel, cobalt, titanium, copper and alloys thereof are
desirable due to high electrical conductivity. Silver in particular
provides excellent electrical conductivity, and commercial silver
nanowires are available. To have good transparency, it is desirable
for the nanowires to have a small range of diameters. In
particular, it is desirable for the metal nanowires to have an
average diameter of no more than about 250 nm, in further
embodiments no more than about 150 nm, and in other embodiments
from about 10 nm to about 120 nm. With respect to average length,
nanowires with a longer length are expected to provide better
electrical conductivity within a network. In general, the metal
nanowires can have an average length of at least a micron, in
further embodiments, at least 2.5 microns and in other embodiments
from about 5 microns to about 100 microns, although improved
synthesis techniques developed in the future may make longer
nanowires possible. An aspect ratio can be specified as the ratio
of the average length divided by the average diameter, and in some
embodiments, the nanowires can have an aspect ratio of at least
about 25, in further embodiments from about 50 to about 5000 and in
additional embodiments from about 100 to about 2000. A person of
ordinary skill in the art will recognize that additional ranges of
nanowire dimensions within the explicit ranges above are
contemplated and are within the present disclosure.
[0038] As noted above the amount of nanowires delivered onto the
substrate can involve a balance of factors to achieve desired
amounts of transparency and electrical conductivity. While
thickness of the nanowire network can in principle be evaluated
using scanning electron microscopy, the network can be relatively
fragile, which can complicate the measurement. In general, the
fused metal nanowire network would have an average thickness of no
more than about 5 microns. However, the fused nanowire networks are
generally relatively open structures with significant surface
texture on a submicron scale, and only indirect methods can
generally be used to estimate the thickness. The loading levels of
the nanowires can provide a useful parameter of the network that
can be readily evaluated, and the loading value provides an
alternative parameter related to thickness. Thus, as used herein,
loading levels of nanowires onto the substrate is presented as
microgram or milligrams of nanowires for a square centimeter of
substrate. In general, the nanowire networks can have a loading
from about 0.1 microgram/cm.sup.2 to about 5 milligrams
(mg)/cm.sup.2, in further embodiments from about 1
microgram/cm.sup.2 to about 2 mg/cm.sup.2, and in other embodiments
from about 5 microgram g/cm.sup.2 (.mu.g/cm.sup.2) to about 1
mg/cm.sup.2. A person of ordinary skill in the art will recognize
that additional ranges of thickness and loading within the explicit
ranges above are contemplated and are within the present
disclosure.
[0039] Electrical conductivity can be expressed as a sheet
resistance, which is reported in units of ohms per square
(.OMEGA./.quadrature. or ohms/sq) to distinguish the values from
bulk electrical resistance values according to parameters related
to the measurement process. Sheet resistance of films is generally
measured using a four point probe measurement or an equivalent
process. In the Examples below, film sheet resistances were
measured using a four point probe, or by making a square using a
quick drying silver paste. The fused metal nanowire networks can
have a sheet resistance of no more than about 200 ohms/sq, in
further embodiments no more than about 100 ohms/sq, and in other
embodiments no more than about 60 ohms/sq. A person of ordinary
skill in the art will recognize that additional ranges of sheet
resistance within the explicit ranges above are contemplated and
are within the present disclosure. In general, sheet resistance can
be reduced by increasing the loading of nanowires, but an increased
loading may not be desirable from other perspectives as described
further below, and the loading is not as significant as achieving
good fusing for improving the sheet resistance.
[0040] For applications as transparent conductive films, it is
desirable for the fused metal nanowire networks to maintain good
optical transparency. In general, optical transparency is inversely
related to the loading, although processing of the network can also
significantly affect the transparency. The optical transparency can
be evaluated relative to the transmitted light through the
substrate. For example, the transparency of the conductive film
described herein can be measured by using a UV-Visible
spectrophotometer and measuring the total transmission through the
conductive film and support substrate. Transmittance is the ratio
of the transmitted light intensity (I) to the incident light
intensity (I.sub.o). The transmittance through the film
(T.sub.film) can be estimated by dividing the total transmittance
(T) measured by the transmittance through the support substrate
(T.sub.sub). (T=I/I.sub.o and
T/T.sub.sub=(I/I.sub.o)/(I.sub.sub/I.sub.o)=I/I.sub.sub=T.sub.film)
While it is generally desirable to have good optical transparency
across the visible spectrum, for convenience, optical transmission
is reported herein at 550 nm wavelength of light. In some
embodiments, the film formed by the fused network has a
transmission at 550 nm of at least 80%, in further embodiments at
least about 85% and in additional embodiments, at least about 90%.
As noted above, the correlation of good optical transparency with
low electrical resistance can be particularly desirable. In some
embodiments with a sheet resistance from 20 ohm/sq to about 150
ohm/sq, the films can have an optical transmission at 550 nm of at
least about 86%, in further embodiments at least about 88% and in
other embodiments from about 89% to about 92%. In one embodiment,
the film can have a sheet resistance of no more than about 75
ohm/sq and a transparency of at least about 85% at 550 nm. In
another embodiment, the film can have a sheet resistance of no more
than about 175 ohm/sq and a transparency of at least about 90% at
550 nm. A person or ordinary skill in the art will recognize that
additional ranges of optical transmission within the explicit
ranges above are contemplated and are within the present
disclosure.
[0041] As described in the Examples below, the processing
approaches described herein result in the fusing of the metal
nanowires. This fusing is believed to contribute to the enhanced
electrical conductivity observed and to the improved transparency
achievable at low levels of electrical resistance. The fusing is
believed to take place at points of near contact of adjacent
nanowires during processing. Thus, fusing can involve end-to-end
fusing, side wall to side wall fusing and end to side wall fusing.
The degree of fusing may relate to the processing conditions. As
described further below, short processing times are believed to
contribute good fusing without degradation of the nanowire
network.
[0042] In general, suitable substrates can be selected as desired
based on the particular application. Substrate surfaces can
comprise, for example, polymers, glass, inorganic semiconductor
materials, inorganic dielectric materials, polymer glass laminates,
composites thereof, or the like. Suitable polymers include, for
example, polyethylene terephthalate (PET), polyacrylate,
polyolefins, polyvinyl chloride, fluoropolymers, polyamides,
polyimide, polysulfones, polysiloxanes, polyetheretherketones,
polynorbornenes, polyester, polyvinyl alcohol, polyvinyl acetate,
acrylonitrile-butadiene-styrene copolymer, polycarbonate,
copolymers thereof, mixtures thereof and the like. Furthermore, the
material can have a polymer overcoat placed on the fused metal
nanowire network, and the overcoat polymers can comprise the
polymers listed for the substrates above. Moreover, other layers
can be added on top or in between the conductive film and substrate
to reduce reflective losses and improve the overall transmission of
the stack.
Processing of Nanowire Networks
[0043] The improved electrical conductivity and optical
transparency has been found to be obtained with short time
treatment of as deposited metal nanowire films with compounds
comprising halogen anions. Desirable increases in electrical
conductivity have been achieved with both vapor delivery of the
fusing composition or with solution based delivery. The fusing
achieves low electrical surface resistance while maintaining high
levels of optical transmission.
[0044] The formation of the metal nanowire network comprises the
formation of a dispersion of the metal nanowires in a suitable
liquid and applying the dispersion as a coating onto the selected
substrate surface. The concentration of the dispersion can be
selected to obtain a good dispersion of the nanowires to provide
for a desired degree of uniformity of the resulting coating. In
some embodiments, the coating solution can comprise from about 0.1
wt % to about 5.0 wt % metal nanowires, and in further embodiments
from about 0.25 wt % to about 2.5 wt % metal nanowires. A person of
ordinary skill in the art will recognize that additional ranges of
metal nanowire concentrations within the explicit ranges above are
contemplated and are within the present disclosure. Similarly, the
liquid for forming the dispersion can be selected to achieve good
dispersion of the nanowires. For example, alcohols, such as ethanol
or isopropyl alcohol, ketone based solvents, such as methyl ethyl
ketone, organic coating solvents, such as toluene or hexane, or the
like or mixtures thereof, are generally good dispersants for metal
nanowires.
[0045] Any reasonable coating approach can be used, such as dip
coating, spray coating, knife edge coating, bar coating, Meyer-rod
coating, slot-die, gravure, spin coating or the like. After forming
the coating with the dispersion, the nanowire network can be dried
to remove the liquid. The dried film of metal nanowires can then be
processed to achieve nanowire fusing.
[0046] A first approach to fusing can be performed with acid halide
vapor, such as vapor from HCl, HBr, HI or mixtures thereof. HF can
also be used, but HF may be corrosive to some substrate materials
and is more toxic. Specifically, the dried coating can be exposed
to the vapor of the acid halide for a brief period of time. The
hydrogen halide compounds are gaseous and are soluble in water and
other polar solvents such as alcohol. Generally, the vapor for
fusing the metal nanowire film can be generated from a gas
reservoir or from vapor given off by solutions of the hydrogen
halide compounds. Acidic vapors can quickly be passed over the
coating surfaces for example for about 10 s to form the nanowire
network. In general, the coating containing the nanowires can be
treated with acid vapor for no more than about 4 minutes, in
further embodiments for from about 2 seconds to about 3.5 minutes
and in other embodiments from about 5 seconds to about 3 minutes. A
person of ordinary skill in the art will recognize that additional
ranges of treatment times are contemplated and are within the
present disclosure.
[0047] In further embodiments, the initial metal nanowires can be
fused with a solution comprising halide anions. In particular, the
solution comprising dissolved acid halide, dissolved metal halide
salts or a combination thereof. Suitable compositions for forming
the halide solutions include, for example, HCl, HBr, HF, LiCl, NaF,
NaCl, NaBr, NaI, KCl, MgCl.sub.2, CaCl.sub.2, AlCl.sub.3,
NH.sub.4Cl, NH.sub.4F, AgF, or a combination thereof. In particular
NaCl, NaBr, and AgF provide particularly desirable fusing
properties. In general, the halide fusing solution can be added to
a previously formed coating comprising the metal nanowires to fuse
the metal nanowires. Additionally or alternatively, the halide
composition can be combined with the metal nanowire dispersion that
is then deposited as a coating so that the metal nanowires and the
fusing agent are simultaneously deposited. If the fusing agent is
included with the metal nanowires in the metal nanowire dispersion,
a separate fusing solution can also be delivered onto the metal
nanowire coating to add an additional quantity of fusing agent.
[0048] The solutions for separate application of the fusing agent
generally comprise halide ions at concentrations of at least about
0.01 mM, in some embodiments, from about 0.1 mM to about 10M, in
further embodiments from about 0.1 M to about 5 M. The metal
nanowires can be contacted with the halide solution using any
reasonable approach such as dip coating, spraying, or the like.
Alternatively or additionally, the halide salt or acid can be added
directly to dispersant of nanowires in ranges from 0.01 mM to about
1M to form a nanowire and halide mixture. The mixture is then
coated onto the substrate surface as described above to form a
coating. The film formation process then results in the direct
formation of the film with the fusing agent already present.
Whether the solution comprising halide anions is delivered with the
metal nanowire coating solution, with a separate fusing solution or
both, the nanowires in the coating form fused nanowire networks
upon solvent removal and the saturation of the halide ions.
Formation of the nanowire network is complete when the solvent is
completely removed from the coating to form a dry film, and while
not wanting to be limited by theory, the fusing process is believed
to be related to the concentration of the halide anions during the
drying process. A person of ordinary skill in the art will
recognize that additional ranges of concentration within the
explicit ranges above are contemplated and are within the present
disclosure.
[0049] After completing the fusing process, the fused metal
nanowire networks are ready for any additional further processing
to form the final product. For example, the coating or film
containing the metal nanowire networks may be rinsed to remove
unreacted sintering agents, and/or may be encapsulated with a
protective coating. Due to the high transparency with low
electrical resistance, the fused nanowire networks are well suited
for the formation of transparent conductive electrodes, transparent
composites, which can be used for solar cells, displays, touch
screens, solar windows, capacitive switches, and the like.
EXAMPLES
[0050] Silver nanowires with different sizes purchased from either
ACS Materials or Seashell Technology, LLC (CA, USA) were used in
the following examples. The properties of the silver nanowires were
an average diameter of 60 nm and an average length of 10 microns or
an average diameter of 115 nm and an average length of 30
microns.
Example 1
Fabrication of Transparent Conductive Material Using HCl Vapor
Treatment
[0051] This example demonstrates the ability to use a vapor based
fusing agent to chemically drive the fusing of silver nanowires to
dramatically improve the electrical conductivity.
[0052] Commercially available silver nanowires (AgNWs) were
dispersed in alcohols e.g. ethanol or isopropanol to form an AgNWs
dispersion. The AgNWs dispersions were typically in the 0.1-1.0% wt
range. The dispersion was then deposited on glass or polyethylene
terephthalate (PET) surfaces as an AgNWs film using a spray coating
or a hand-drawn rod approach. The AgNWs film was then exposed
briefly to HCl vapour as a fusing agent. Specifically, the AgNWs
film was exposed to HCl vapour from a concentrated HCl solution at
room temperature for about 10 seconds. AgNWs from two different
vendors were used. The sheet resistance and transparency of the
AgNWs film before and after the treatment with HCl vapour were
measured and recorded. The data of AgNWs from the first vendor is
listed in Table 2 and the date of AgNWs from the second vender is
listed in Table 3 below.
TABLE-US-00001 TABLE 2 Sample Sheet Resistance Sheet Resistance No.
Before HCl (ohm/sq) After HCl (ohm/sq) 1 10000000 660 2 83000 60 3
10000000 1909 4 10000000 451 5 800000 113.4 6 695000 30 7 10000000
62 8 399000 562 9 14,200 53.4 10 10000000 283 11 10000000 1260 12
10000000 364 13 10000000 6700 14 10000000 1,460 15 10000000 70.5 16
10000000 2280 17 10000000 155 18 10000000 1654 19 10000000 926
TABLE-US-00002 TABLE 3 Sheet Resistance Before HCl Sheet Resistance
After HCl Sample (ohm/sq) (ohm/sq) 1 13180 253 2 6200000 244 3 6030
115 4 32240 43.6 5 4300000 68.3 6 10000000 1060 7 10000000 47.5 8
3790 61.7 9 4690 42.4 10 404 37.5
[0053] Because the large numerical range involved, the data were
plotted in logarithmic format in figures so the small numbers can
also be visualized graphically. The data from table 2 was plotted
in FIG. 2 and data from table 3 was plotted in FIG. 3. The films
corresponding to the electrical conductivity results in Tables 2
and 3 had moderate loadings with corresponding reasonable
transparency to visible light. As shown in FIG. 2, the conductivity
of the AgNWs film improved over 4 to 5 orders of magnitude after
the HCl vapour treatment. Additionally, these AgNWs films showed
transparencies at 550 nm greater than 75%, which decreased less
than 0.5% after HCl vapor treatment. Similarly, in FIG. 3, dramatic
improvement in conductivity was also observed. The properties of
the nanowire networks after fusing were relatively independent of
the properties of the initial nanowires for these two sets of
nanowires, but the longer nanowires exhibited overall a reduced
electrical resistance prior to fusing.
[0054] Additional AgNWs films were formed that has transparencies
at 550 nm greater than 85%. These films were also treated with HCl
vapor for about 10 seconds, and the sheet resistances of the AgNWs
films before and after the HCl vapour treatment were measured. The
results for one set of samples are presented in Table 4, and
results for another set of samples are plotted in FIG. 4. Samples
2, 3, and 4 in FIG. 4 in particular have sheet conductivity between
30 to 50 ohm/sq while maintaining the transparency of the films
above 85%. The results shown in Table 4 clearly demonstrate the
ability to obtain transmission with 550 nm light greater than 90%
with sheet resistance values less than 50 ohm/sq.
TABLE-US-00003 TABLE 4 Resistance Prior to Resistance After
Transmission at 550 nm Sintering Sintering (Conductive Film Only)
801 45 89.1 >10.sup.6 40 88.9 >10.sup.6 33 88.1 >10.sup.6
20 87.8 >10.sup.6 46 90.6 >10.sup.6 182 92.4 >10.sup.6 129
91.6 >10.sup.6 85 89.2
Example 2
Observation of the Fusing of the Silver Nanowires
[0055] This example provides evidence of nanowire physical fusing
as a result of contact with chemical fusing agents.
[0056] The dramatic conductivity improvement observed in Example 1
can be attributed to the fusing of some of the silver nanowires
with adjacent silver nanowires. Scanning electron micrographs (SEM)
of the silver nanowires before treatment were obtained and are
shown in FIG. 5. As shown in FIG. 5, some of the ends (indicated by
the circles) of the silver nanowires appear to touch each other,
but the ends apparently do not appear to be fused together.
Additionally, polyvinylpyrrolidone (PVP) coating (indicated by
arrows in the figure) can be seen to be present around the rods. As
a comparison, the silver nanowires shown in FIG. 5 were heated at
100.degree. C. for 10 minutes. No appreciable conductivity change
has been observed after the heating. SEM micrographs of the silver
nanowires after the heat treatment were obtained and are shown in
FIG. 6. Heating does not appear to have fused the ends as shown in
FIG. 6, some of the ends (indicated by the circles) of the silver
nanowires do not appear to be fused together. Scanning electron
micrographs were obtained for nanowire networks after the HCl vapor
treatment and are shown in FIG. 7. SEM of the silver nanowires in
FIG. 7 after the HCl treatment showed the ends (indicated by the
circles) of the silver nanowires have been fused together, and
other locations of contact between adjacent nanowires are believed
to similarly fuse to form fused silver nanowire networks.
Example 3
Fabrication of Transparent Conductive Material Using Halide
Solution Treatment
[0057] This example demonstrated the reduction in electrical
resistance through the treatment of the networks with solutions
containing halide anions.
[0058] Specifically, 50 mM solutions of AgF or NaCl in ethanol were
used to treat the AgNWs films. When the fusing agent solution is
used, the AgNWs film was submerged or covered with the fusing agent
solution for about 10 to about 30 seconds, or dilute solutions of
AgF or NaCl were spray coated (from ethanol) onto the AgNW. The
AgNWs were then allowed to dry. The sheet resistance of the AgNWs
film before and after the treatment with the halide solutions were
measured and the results are shown in FIG. 8. As shown in FIG. 8
dramatic conductivity improvement is also observed of the AgNWs
films treated with halide solutions, with AgF treated samples
showing even more pronounced improvement compared to the NaCl
treated samples. In general, the transmission of light changed
marginally (<5%) and less than 1% if residual salt solution was
removed. Residual salt was removed by spraying gently with water or
ethanol.
[0059] Dramatic improvements in conductivity with negligible
changes in transparency are important for transparent conductor
applications. The conductivity of transparent conductors is often
improved by adding more conducting materials, for example more
AgNWs, but the transmission can dramatically decrease. The methods
and processes described herein provide a convenient and cost
effective approach to dramatically improve the conductivity of
nanowire materials without sacrificing transparency or adding
additional nanowires.
[0060] The embodiments above are intended to be illustrative and
not limiting. Additional embodiments are within the claims. In
addition, although the present invention has been described with
reference to particular embodiments, those skilled in the art will
recognize that changes can be made in form and detail without
departing from the spirit and scope of the invention. Any
incorporation by reference of documents above is limited such that
no subject matter is incorporated that is contrary to the explicit
disclosure herein.
* * * * *